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As impressive as the correlation data are the test results themselves. Figures from the British National Survey of Health and Development have been published by Douglas (35). The performance of the children studied on the Junior Leaving, or 11-plus, Examination is shown in Table 9, together with their test results at 8 years of age. The expected social class differences in performance are already evident in the younger children, but equally evident at both ages and in all social classes are marked differences associated with family size. It should be noted further that whereas the effects of family size on growth were ameliorated by social class in this same population (see Table 5), such was not the case with intelligence test performance.
Another study has shown the persistence of these differences into adulthood. Vernon (43) analyzed the data from about 10,000 British male National Service Recruits and examined the relation between number of
TABLE 9
Average Intelligence Test Scores1 by Completed Family Size and Social Class, Great Britain 1954-1957
Completed Family Size
Social Class	Age at Test	1	2	3	4	5	6	7 or More	Unknown
Upper middle	11 8	59.87 59.20	57.31 56.82	55.80 55.44	56.49 56.79	55.65 54.60	54.45 52.14	54.00 54.33	55.00 63.50
Lower middle	11 8	54.60 53.88	55.27 54.26	53.20 52.64	52.02 50.20	51.81 50.03	50.11 51.43	47.81 47.95	59.00 58.00
Upper manual working	11 8	52.74 52.27	52.19 51.64	49.90 49.93	48.61 48.65	47.40 47.31	45.80 48.53	40.54 42.49	47.50 44.00
Lower manual working	11 8	50.93 51.54	48.71 49.64	48.16 48.38	46.64 47.44	45.78 45.27	44.86 45.51	42.19 42.44	44.73 45.09
All social classes	11 8	52.96 52.86	52.16 52.09	50.41 50.36	48.57 48.74	47.51 46.87	46.04 47.07	42.49 43.06	47.17 47.00
Social class held constant	11 8	52.87 52.83	51.63 51.69	50.27 50.23	49.06 49.07	48.26 47.53	46.97 48.03	43.98 44.61	49.31 49.14
aThese are "T scores which were designed so that the average score for all children in the population is fifty and the standard deviation 10. ... To convert T scores into I.Q.'s the following formula may be used:
I.Q. =  25 + 1.5 (T score)." (34, pp. 34-35) Source:  Douglas (35).he negative correlation between intelligence and family size varied among the classes. It was "clearly apparent" in children of fanners, manual laborers, and clerical workers, "negligible" in children from the managerial class, and "barely discernible" among those from the professional class.